1. Field of the Invention
The present invention relates to an image processing apparatus for processing an image in digital manner.
2. Related Background Art
Conventional analog copying machines are being replaced by so-called digital copying machines, in which an original image is sampled for example with a CCD sensor, and the image is reproduced from the digitized data through a digital printer such as a laser beam printer, owing to the progress in digital devices.
In such digital copying machines, a dither method or density pattern method is employed for reproducing intermediate tones. Such methods have however been associated with following drawbacks:
(1) If the original image has screen dots, as in a printed image, the copied image shows periodic stripe patterns not present in the original image; and
(2) If the original image contains lines or characters, the edges of such lines or characters are broken by the dither process, and the image quality is deteriorated.
The phenomenon (1), called Moire patterns, arises from:
(A) beats between the screen dots of the original image and the sampling operation; or
(B) beats between the screen dots of the original image and the dither threshold matrix.
The phenomenon (B) is particularly apparent when the dither threshold values are arranged in a dot concentrated pattern. In such case the output image has a pseudo-screen dot pattern, which generates beats with the screen dots of the original image, thus leading to a Moire phenomenon.
For digitization of an original image, there is also known an error dispersion method, in which the difference between the density of the original image and the density of the output image is calculated for each pixel, and the error obtained from said calculation is dispersed to the surrounding pixel with certain weighting. This method was reported by R. W. Floyd and L. Steinberg in "An Adaptive Algorithm for Spatial Grey Scale", SID 17, PP. 75-77 (1976). This method does not generate Moire patterns in the reproduction from a screen dot image, as it lacks periodicity in the process, in comparison with the dither method. However, it is still associated with certain drawbacks such as stripe patterns in the output image specific to this method, and granular noise in the highlight areas or dark areas of the image.